An important class of mass air flow sensors is based on the measurement either of the amount or the time of flight of heat transported by the air flow between a resistive heater and a temperature difference detector formed by a thermocouple. An example of such a mass air flow sensor is described in U.S. Pat. No. 4,576,050.
In sensors of this kind, a suitable common substrate supports both a resistive strip heater and a thermoelectric detector or thermocouple spaced apart a fixed distance in a parallel relationship. An oscillator supplies oscillating current to the strip heater to create similar temperature variations in the heater that are used to heat correspondingly the air flowing past the strip normal to its longitudinal axis for continued flow past the thermoelectric detector.
In particular, the detector generally comprises a thin continuous film of material, having a high thermoelectric power and low resistivity, that is positioned between a pair of elongated parallel strips to form a pair of spaced elongated junctions that form the thermocouple of the detector. Temperature differences between the two junctions give rise to a voltage which is a measure of the difference. A differential amplifier having its inputs coupled to the two strips will produce an output signal when a temperature difference exists. Any of a variety of known techniques can be used to separate out this output signal. Typically, as described in the aforementioned patent, this involves filtering the output signal to pass a frequency twice that of the frequency of the oscillator since each sine wave cycle of the oscillator will produce two heat pulses in the strip heater and give rise to two output pulses. The filtered output signal and a signal from the oscillator are then fed to a circuit which is sensitive to the time differential or phase difference between the oscillator current and the detector output signal. This time differential or phase difference will be a measure of the velocity of the air flow past the two junctions of the detector.
For use in sensors of this kind, high and stable sensitivity is needed for the detector as well as compact size. In particular, there is needed a detector which in size and power is readily compatible with silicon integrated circuitry since such circuitry advantageously is used in conjunction with the detector to make a complete sensor system.